The main aspects of the prior art oscillator circuit 10 shown in FIG. 1 have been described in U.S. Pat. No. 7,053,722 B2. The circuit 10 comprises as its core element a VCO circuit (VCO core) 8 for generating an oscillatory voltage between output terminals 56 and 56′. The VCO core comprises a bipolar transistor 30 the base of which is coupled to an inductor 32 and the emitter of which is coupled to a capacitor 18. The capacitor 18 is implemented here as a varactor (variable capacitance diode), tunable by means of a tuning voltage at a tuning terminal 22. The circuit is symmetric with respect to a symmetry axis passing through virtual ground nodes 60, 34, 44 at which there is no fundamental current. The VCO core is powered by means of a supply voltage provided between a ground terminal 12 and a DC voltage supply terminal 28. The VCO core 8 is coupled to the ground terminal 12 via a current source 14 and at the node 60 to the supply terminal 28 via a non-essential forward-sense diode 62. The diode 62 mainly serves to somewhat lower the potential at the node 60 with respect to the supply potential provided at the terminal 28. A number of supplementary inductors 58, 54, 50, 46, and their “mirror” counterparts 58′, 54′, 50′, 46′ complete the resonator 8.
An inductor 64 is shown between the supply terminal 28 and the diode 62. It represents the inductance of the bond wires by which the oscillator circuit, situated on a chip, is connected to the external supply terminal 28. The bond wire inductance 64 is an undesired effect, as it allows for spurious high-frequency signals to be present on the common power-line comprising nodes 52, 66, 70 serving to supply a voltage not only to the VCO core circuit but also to a subsequent circuit 6 comprising a buffer 68 and a two-stage power amplifier (PA) 74 coupled in series. The buffer 68 takes the differential voltage output at the VCO core's output terminals 56 and 56′ and passes it on to the amplifier 74, which produces an amplified voltage or current between the load terminals 78 and 80 for driving an antenna or other subsequent circuit (not shown) coupled between the terminals 78 and 80. The buffer 68 and the PA 74 also improve the decoupling between the oscillator core 8 and the extended load by attenuating what is known as the pulling effect. However, the buffer 68 and the PA 74 nearly always work in large signal conditions and therefore they generate many harmonic signals. These harmonic signals can easily enter the resonator circuit through the voltage supply line, passing through the nodes 72, 70, 66, 52 and 60 to the resonators differential output terminals 56, 56′, thereby increasing the resonators 8 phase noise by a down-conversion of these signals into the oscillator signal.